1 66 



POPULAR SCIENCE MONTHLY. 



and a supply of air from the surface is out of the question, recourse is 

 stili had as before to some form of storage power for propulsion. At 

 present this is always electric. 



The problem of diving demands attention next. For surface sailing 

 a submarine boat, like any other, needs considerable buoyancy, so as 

 to float with a considerable fraction of its bulk free above water. For 

 diving, on the other hand, her buoyancy must be very small. These 

 conditions are met by varying the amount of ballast carried. This is 

 universally done by admitting water into, or expelling it from, suitable 

 air-tight tanks distributed through the bottom of the boat. The filling 

 of these tanks recuircs only the opening of a valve. To empty them 



' , "*wp* s ""» 



Fig. 6. Photographs of a Trial of the 'Holland,' showing her in Cruising Trim, 

 in Diving Trim, Diving, and Rising after the Dive. 



requires power. Formerly this was done by means of pumps. But 

 pumping is slow work. A much more expeditious method of emptying 

 the water tanks is to blow out the water by admitting compressed air 

 from the reservoirs. The air so used is finally delivered into the living 

 rooms for breathing, and the pressure in the reservoirs is restored 

 again win n at the surface. By thus varying the quantity of ballast a 

 boat may be caused to sink, or, if already beneath the water, be caused 

 to rise to the surface either slowly or rapidly as may be desired. It 

 is easy to imagine circumstances, either accidental or otherwise, where 

 a very sudden return to the surface might be imperative. To provide 

 for this in emergencies the most practical boats are furnished with 



